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Electroless Ni-P metallization on palladium activated polyacrylonitrile (PAN) fiber by using a drying process
Materials Chemistry and Physics 204 (2018) 257e261
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electroless Ni-P metallization on palladium activated polyacrylonitrile (PAN) fiber by using a drying process Jae-Young Lee*, Hong-Ki Lee Hydrogen Fuel Cell Parts and Applied Technology Regional Innovation Center, Woosuk University, Jeollabuk-do 55315, South Korea
h i g h l i g h t s A drying process is proposed to activate PAN fiber surface with Pd nanoparticles. Ni-P alloy is electrolessly plated on the Pd activated PAN surface. 1.22 mm thick Ni-P alloy was covered the PAN fiber. EMI shielding effectiveness was over 30 dB at 30e1500 MHz.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2016 Received in revised form 25 September 2017 Accepted 17 October 2017
Polyacrylonitrile (PAN) fiber was metallized by nickel-phosphorus (Ni-P) in an electroless plating process. Firstly, the surface of PAN fiber was activated by palladium (Pd) nanocatalysts by using a drying process in which palladium(II) bis(acetylacetonate), Pd(acac)2 was spontaneously reduced to Pd nanocatalysts in180 C N2 atmosphere without using any reducing agent. And then Ni-P alloy was coated on the Pd activated PAN fiber in an electroless Ni-P plating solution without using SnCl2-sensitizing solution. The morphology was observed by scanning electron microscopy (SEM) and elemental analyses were carried out by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The composition ratio of Ni: P was 85.3: 14.7 atomic % and the growth rate of Ni-P layer decreased with increasing plating time. The thickness of Ni-P plating layers for 10, 30 and 60 min were 0.64, 1.03 and 1.22 mm, respectively. Electromagnetic interference shielding effectiveness (EMI-SE) of Ni-P plated PAN fiber (50 wt%)/epoxy composite was more than 30 dB in the frequency range from 30 MHz to 1500 MHz. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ni-P fiber Electroless plating Palladium nanocatalyst PAN fiber Drying process
1. Introduction As automotive and electronic industries have been growing rapidly, electromagnetic interference (EMI) and radio-frequency interference (RFI) shielding materials have become important [1e4]. Especially, polymer composites with discontinuous conducting fillers such as metal particles, metal flakes, carbon particles and carbon fibers are widely used for EMI shielding and the effectiveness increased with increasing volume fraction of the filler and with increasing aspect ratio of the fillers [5e7] because they are more attractive as compared to monolithic metals in terms of weight and processability. Z. Durmus et al. [1] reported that novel type graphene/barium hexaferrite nanocomposites were synthesized via a citrate solegel combustion method to prepare
* Corresponding author. E-mail address: [email protected] (J.-Y. Lee). https://doi.org/10.1016/j.matchemphys.2017.10.048 0254-0584/© 2017 Elsevier B.V. All rights reserved.
microwave-absorbing material. P. Latko et al. [2] showed that polyamide 11/multi-walled carbon nanotube nanocomposite fibers were prepared and they found that the nanocomposite fiber could be used as a precursor for non-woven or woven fabrics to manufacture EMI shielding material. X. Shui and D. D. L. Chung [3] reported that polyethersulfone/nickel filament (0.4 mm diameter) composite had high electromagnetic interference shielding effectiveness, high reflection coefficient and low surface impedance at 1e2 GHz. Z. Zhang et al. [4] showed that electrical conductive nanocomposite fibers were prepared with polyaniline (PANI), polyacrylonitrile (PAN) and multi-walled carbon nanotubes (MWCNTs) via electrospinning and the electrical conductivity of nanocomposite fibers increased from 1.79 S m1 to 7.97 S m1 with increasing the MWCNT content from 3.0 wt% to 7.0 wt%. To deposit metallic particles on the plastic surface, metallization processes were carried out through physical deposition method or wet chemical deposition method [8e12]. A. M. Abdel Reheem et al.
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[8] reported on the copper films deposited onto polycarbonate (PC) substrate after surface modification using low argon ion beam and Z. Sun et al. [9] showed an electroless nickel plating method deposited on a primer-printed polyethylene terephthalate (PET) surface by immersing in 3-aminopropyltriethoxysilane solution. S. Olivera et al. [10] showed that a plating technique on acrylonitrilebutadiene-styrene (ABS) surface can serve to enhance the strength and structural integrity as well as to improve durability and thermal resistance, and H. N. Zhang et al. [11] reported that copper was plated on the ABS surface in an electroless copper solution, in which the ABS surface was previously modified by heterocyclic organosilane self-assembled film. Wet chemical deposition methods are usually classified into two categories: electroplating process and electroless plating process. However the former can't be applied to polymer substrates because polymers can't pass electric current. Therefore polymer materials are deposited by using an electroless plating process in general. Prior to electroless deposition, the polymer surface is pre-treated by physical and chemical modification methods and there are many pre-treatment methods: chemical, plasma, laser and catalytic activation methods [12], and in this study catalytic activation method was used. The most commonly used catalyst is palladium (Pd) nanocatalyst because it has good activation property compared to other catalysts such as copper, gold, silver or aluminum nanocatalysts [13]. To activate the polymer surface with Pd nanocatalyst, a combination of Sn and Pd chemistry process is widely used and there are two main processes: one is a two-step process which uses successive dilute solutions of SnCl2 and then PdCl2 [14,15], and the other is a one-step process which uses a SnCl2ePdCl2 mixed solution [14,16]. In recent the latter one, so-called Pd/Sn mixed catalyst is most widely used process in which the mixed catalyst is prepared by mixing PdCl2 with large excess of SnCl2 in HCl solution. Another method to activate the polymer surface with Pd nanocatalyst is to use organometallic compounds such as palladium(II) acetylacetonate (Pd(acac)2), palladium(II) dimethyl sulfoxide (Pd-DMSO), palladium(II) hexafluoroacetylacetonate (Pd(hfa)2), which are dissolved in supercritical CO2 solution [17,18]. We also reported a Pd activation method from Pd(acac)2 on the surface of polymers by using a drying process [19e21]: Firstly, Pd(acac)2 was sublimed and penetrated into polymer surface at 180 C in vacuum condition, and spontaneously reduced forming Pd nanocatalysts. And then Ni-P or ZnO plating was site-selectively deposited on Pd activated surface. In this study PAN fiber surface was activated by Pd nanocatalysts via drying process and Ni-P plated PAN fiber with long aspect ratio was prepared in electroless Ni-P solution. 2. Experiment Pd(acac)2 was purchased from Johnson Matthey Materials Technology, and it was recrystallized in boiled acetone. PAN fiber was obtained from Tai'an Pole Geosynthetics Co., Ltd. (China), whose diameter was 13 ± 1 mm and the length was 15 mm. Its tensile strength was 910 MPa, initial modulus was 100 GPa and density was 1.18 g/cm3. The purchased PAN fiber was cleaned with acetone in order to remove contaminated oil spots during handling and was dried in a vacuum oven at 40 C for 48 h before use. Nickel(II) sulfate hexahydrate (NiSO4$6H2O), sodium hypophosphite monohydrate (NaPH2O2$H2O), propionic acid (CH3CH2COOH), lactic acid (CH3CH(OH)COOH), and ethanol were used as received from Wako Pure Chemical Industries, Ltd. In order to activate the surface of PAN fiber with Pd nanocatalyst, one-step drying process was used [19e21]. As shown in Fig. 1, firstly 0.02 mg of Pd(acac)2 was poured into a 50 mL quartz reactor and it sublimated at 180 C oil bath. The sublimated
Pd(acac)2 recondensed on the upper side of the reactor wall, PAN fiber with ca. 0.5 cm length was put into the reactor, and the atmosphere of the reactor was exchanged with N2. If the reactor containing Pd(acac)2 and PAN fiber was kept at 180 C oil bath, the sublimated Pd(acac)2 precursors penetrated into PAN fiber surface and the Pd(acac)2 compounds were spontaneously reduced to Pd nanoparticles in the PAN fiber surface without any reducing agent at 180 C N2 atmosphere. The exposure time of PAN fiber to Pd(acac)2 was 5 min. To remove the unreduced Pd(acac)2 on the PAN surface, the Pd activated PAN fiber was cleaned in boiled acetone and dried in a vacuum oven at 40 C for 24 h. Electroless Ni-P plating solution was prepared by dissolving 6.3 g of nickel(II) sulfate hexahydrate (Ni source), 7.5 g of NaPH2O2$H2O, 7.5 g of lactic acid (buffer and complexant), and 8.1 g of propionic acid (buffer) in 60 mL deionizer water and pH was adjusted to 4.5 ± 0.1 by NaOH aqueous solution. Then, 30 mL ethanol was mixed with the electroless Ni-P plating solution in order to modify wettability of PAN fiber to the electroless solution. 0.5 g of chopped PAN fiber was put into the electroless solution and Ni-P plating was carried out in the plating solution at 80 C for 10, 30 and 60 min. Ni-P plated fiber was cleaned with distilled water and was dried in a vacuum oven at 40 C for 24 h. The dispersion of Pd nanocatalysts on the surface of PAN fiber was observed by a transmission electron microscopy (TEM, Carl Zeiss Co. Ltd., Germany) at an accelerating voltage of 200 kV. Thin sections for Pd/PAN fibers were prepared by cryo-ultramicrotomy at 60 C after embedding in a light curable resin system (D-800, JEOL DATUM, Japan). The morphology of Ni-P plated fiber was observed by a field emission scanning electron microscopy (FESEM, JMS-6701F, JEOL) which was operated at an acceleration voltage of 10 Kv. The elemental analysis was carried out by energydispersive X-ray spectroscopy (EDS, JED-2300 Energy Dispersive Xray Analyzer, JEOL) which was connected to the FE-SEM. X-ray photoelectron spectroscopy (XPS) spectrum was acquired by Thermo Scientific K-Alpha model equipped with 180 double focussing hemispherical analyzer using monochromated Al KAlpha radiation. To measure the tensile strength of a single PAN fiber or a single Ni-P plated fiber, universal testing machine (Shimadzu, Table top type tester EZ-Test) was used with a load cell of 10 N and the crosshead speed was set to 0.2 mm/min. The specimen was prepared by fixing a filament on a paper holder with an instant adhesive, as reported on the other paper [22], and its diameter was measured by using an optical microscope. EMI shielding effectiveness (EMI-SE) of Ni-P plated PAN fiber/ epoxy composite was assessed using coaxial transmission line method according to ASTM D4935 [23]. The composite specimen was prepared by curing the mixture of Ni-P plated PAN fiber (50 wt %) and epoxy matrix at 150 C for 2 h. The Ni-P plated PAN fiber used here was plated at 80 C for 60 min. 3. Results and discussion Fig. 1 shows a TEM image for Pd nanocatalysts activating the PAN fiber surface prepared in 180 C N2 atmosphere for 5 min. Almost all Pd nanocatalysts were located on the surface area of the PAN fiber so that the Pd nanocatalysts could act as nucleus for Ni-P growth. To confirm the formation of Pd nanocatalysts through the reduction of Pd(acac)2, Pd activated PAN fiber was analyzed by EDS and the spectrum was shown in Fig. 2. The peaks for carbon (C) and nitrogen (N) were due to PAN fiber and Au peak was owing to gold coating for SEM observation. There were only Pd peaks without oxygen (O) peak which meant all Pd(acac)2 compounds spontaneously reduced to metallic Pd nanocatalysts. The formation mechanism of metallic Pd nanocatalysts in PAN fiber was divided
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Fig. 1. One-step drying process for the preparation of Pd activated PAN fiber.
Fig. 3. (a) SEM image of Pd activated PAN fiber prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min and (b) EDS data.
Fig. 2. Pd nanocatalysts on the surface of PAN fiber prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min.
into two steps: the first step was nucleation step and the second one was growth step in the PAN fiber surface area. In the nucleation step, Pd(acac)2 as metallic precursors sublimed in 180 C N2 atmosphere, penetrated into the surface of PAN fiber, and the precursors adsorbed on nucleophilic C≡N groups of PAN fiber. And then the precursors spontaneously reduced to Pd elements forming nanocatalysts as nucleation sites. And in the Pd growth step, Pd precursors continuously adsorbed on the nucleation sites and reduced to Pd element resulting fast increasing Pd nanoparticles. In this work, however, Pd nanocatalysts were positioned at the surface area of PAN fiber because PAN fiber was too dense for vaporized Pd(acac)2 to penetrate deeply into the PAN fiber, and it was advantageous to electroless plating. Fig. 3 shows SEM images in order to observe Ni-P morphology plated on PAN fiber surface which was prepared in electroless plating solution at 80 C for 30 min: (a) 350x, (b) 1,600x and (c)
6,500 magnification. As was expected, 0.5 cm PAN fiber was evenly plated with Ni-P alloy as shown in Fig. 3(a), and when the surface was zoomed in nickel clusters grew along the furrows of PAN fiber as shown in Fig. 3(b) and (c). Because surface roughness of polymer affect the adhesion strength between plated layer and the polymer surface, the furrows on the PAN fiber, as shown in Fig. 2(a) would have good effect on the adhesion strength. The electroless mechanism of Ni-P plating could be divided into four elementary steps [24]: (1) reactants (Ni2þ, H2PO 2 etc.) diffused to the surface of PAN fiber, (2) adsorpted on Pd nanocatalysts on the PAN surface, and (3) Ni-P plating reaction took place on the PAN þ surface, and then (4) the by-products (HPO 3 , HI, H etc.) desorpted and diffused from the PAN surface. In these steps, the first step was rate determining step because the surface of PAN fiber was
Fig. 4. SEM images of Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 30 min: (a) 350x, (b) 1,600x and (c) 6,500 magnification.
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hydrophobic, therefore ethanol was mixed into the conventional Ni-P plating solution in order to improve the wettability of PAN fiber to the plating solution in order to increase the reaction rate of the first step. Fig. 4 shows EDS spectrum for Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 30 min. The morphology of
Fig. 5. (a) SEM image of Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 30 min and (b) EDS data.
Fig. 6. XPS survey spectra for Pd activated PAN fiber prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min and Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 10 min.
Ni-P plated PAN fiber was observed by SEM as shown in Fig. 4(a) and the elemental composition was analyzed by EDS as shown in Fig. 4(b) resulting the elemental composition of Ni: P was 94.3: 5.7 atomic %. There was no atomic peak for nitrogen or carbon atom owing to PAN fiber and this meant the surface of PAN fiber was wholly covered by Ni-P alloy. Fig. 5 shows XPS survey spectra for Pd activated PAN fiber and for Ni-P plated PAN fiber. Pd activation of PAN fiber was prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min and Ni-P plated PAN fiber was prepared at 80 C for 10 min in electroless plating solution. In the spectrum of Pd activated PAN fiber, if displayed in detail, the binding energy (BE) values of Pd3d peaks would be shown at 341.1 eV and 335.5 eV [20], and the BE values of C1s and N1s were 286.2 eV and 398.0 eV, respectively for the nitrile group of PAN fiber [25]. This XPS spectrum confirmed that all the penetrated Pd(acac)2 compounds were reduced to Pd metallic nanoparticles because there was no O1s peak owing to Pd(acac)2 compound. However, in the spectrum of Ni-P plated PAN fiber, the BE values of C1s and N1s were reduced very significantly and new peaks for nickel (855.7 and 870.7 eV) and phosphorus (135.2 eV) elements. These meant that almost all the surface of PAN fiber was covered by Ni-P alloy, and a weak peak of C1s was due to the carbon atoms at the open hole on the PAN surface as shown in Fig. 6(a). As Ni-P plating time was 30 min, the peak of C1s was completely disappeared, although its XPS spectrum was not displayed in Fig. 5, and this meant that all the surface of PAN fiber was completely covered by Ni-P deposition as shown in Fig. 6(b). The effect of electroless plating time on Ni-P thickness was studied by using SEM observation shown in SEM images in Fig. 6, in which each Ni-P was plated on PAN fiber in electroless plating solution at 80 C for (a) 10, (b) 30 and (c) 60 min, and each prime (’) image was prepared by thermal decomposition of each Ni-P plated fiber at 600 C for 30 min in N2 atmosphere condition in order to measure Ni-P thickness. As was reported by others [26,27], Ni-P particles generated irregularly on the PAN surface and grew gradually. Almost all the PAN surface was covered in 10 min reaction as shown in Fig. 6(a), and as electroless plating time increased, the thickness of Ni-P increased and the morphology of Ni-P surface became rough. The Ni-P thickness in 10 min was 0.64 mm and those in 30 and 60 min were 1.03 and 1.22 mm, respectively. The growth rate of Ni-P layer decreased with increasing plating time. Tensile strength of a single PAN fiber was 918.6 MPa and its initial modulus was 125.7 GPa, which were was in good agreement
Fig. 7. SEM images of Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for (a) 10, (b) 30 and (c) 60 min. Each prime (’) image was prepared by thermal decomposition of Ni-P plated PAN fiber at 600 C for 30 min in N2 atmosphere condition in order to measure Ni-P thickness.
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time was 10 min, almost all the PAN surface was evenly covered by Ni-P alloy with some open holes and the holes were completely filled as the plating time became 30 min. Ni-P alloy thickness was 0.64, 1.03 and 1.22 mm in the reaction time of 10, 30 and 60 min, respectively. EDS analysis showed that elemental composition ratio of Ni: P was 85.3: 14.7 atomic %. EMI-SE of Ni-P plated PAN fiber (50 wt%)/epoxy composite was more than 30 dB in the frequency range from 30 MHz to 1500 MHz, therefore the composite could be used for EMI shielding materials.
Acknowledgement This work was financially supported by the Regional Innovation Center (RIC) program of the Ministry of Trade, Industry and Energy (MOTIE No. R0010583), Korea in 2016 and by the Ministry of Education (MOE No. 2017CG0260101) through the Leades INdustryuniversity Cooperation Project.
References
Fig. 8. EMI shielding effectiveness of Ni-P plated PAN fiber (50 wt%)/epoxy composite where the Ni-P plated PAN fiber used here was plated at 80 C for 60 min.
with the specifications provided by the manufacture. Those values for Ni-P plated PAN fiber, which was prepared at 80 C for 60 min were 895.2 MPa and 138.4 GPa, respectively. Generally, when the metal was coated, the tensile strength increased, however the value slightly decreased in here. This was probably because the structure of the electrolessly plated Ni-P was not dense, as shown in the SEM image of Fig. 7. Fig. 8 showed the EMI-SE of Ni-P plated PAN fiber (50 wt %)/epoxy composite where the Ni-P plated PAN fiber used here was plated at 80 C for 60 min. It showed that the EMI-SE was more than 30 dB in the frequency range from 30 MHz to 1500 MHz. Therefore, the composite could be practically useful for many applications requiring EMI shielding [23]. 4. Conclusions Ni-P metallized PAN fiber was prepared by using electroless plating process, in which the surface of PAN fiber was previously activated by Pd nanocatalyst via the spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min. TEM observation showed that almost all Pd nanocatalysts were located on the surface area of the PAN fiber and SEM observation showed that Ni-P alloy clusters grew along the furrows of the Pd activated PAN fiber surface. As plating time increased, the Ni-P layer thickness became thick while the growth rate decreased. When Ni-P plating
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electroless Ni-P metallization on palladium activated polyacrylonitrile (PAN) fiber by using a drying process Jae-Young Lee*, Hong-Ki Lee Hydrogen Fuel Cell Parts and Applied Technology Regional Innovation Center, Woosuk University, Jeollabuk-do 55315, South Korea
h i g h l i g h t s A drying process is proposed to activate PAN fiber surface with Pd nanoparticles. Ni-P alloy is electrolessly plated on the Pd activated PAN surface. 1.22 mm thick Ni-P alloy was covered the PAN fiber. EMI shielding effectiveness was over 30 dB at 30e1500 MHz.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2016 Received in revised form 25 September 2017 Accepted 17 October 2017
Polyacrylonitrile (PAN) fiber was metallized by nickel-phosphorus (Ni-P) in an electroless plating process. Firstly, the surface of PAN fiber was activated by palladium (Pd) nanocatalysts by using a drying process in which palladium(II) bis(acetylacetonate), Pd(acac)2 was spontaneously reduced to Pd nanocatalysts in180 C N2 atmosphere without using any reducing agent. And then Ni-P alloy was coated on the Pd activated PAN fiber in an electroless Ni-P plating solution without using SnCl2-sensitizing solution. The morphology was observed by scanning electron microscopy (SEM) and elemental analyses were carried out by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The composition ratio of Ni: P was 85.3: 14.7 atomic % and the growth rate of Ni-P layer decreased with increasing plating time. The thickness of Ni-P plating layers for 10, 30 and 60 min were 0.64, 1.03 and 1.22 mm, respectively. Electromagnetic interference shielding effectiveness (EMI-SE) of Ni-P plated PAN fiber (50 wt%)/epoxy composite was more than 30 dB in the frequency range from 30 MHz to 1500 MHz. © 2017 Elsevier B.V. All rights reserved.
Keywords: Ni-P fiber Electroless plating Palladium nanocatalyst PAN fiber Drying process
1. Introduction As automotive and electronic industries have been growing rapidly, electromagnetic interference (EMI) and radio-frequency interference (RFI) shielding materials have become important [1e4]. Especially, polymer composites with discontinuous conducting fillers such as metal particles, metal flakes, carbon particles and carbon fibers are widely used for EMI shielding and the effectiveness increased with increasing volume fraction of the filler and with increasing aspect ratio of the fillers [5e7] because they are more attractive as compared to monolithic metals in terms of weight and processability. Z. Durmus et al. [1] reported that novel type graphene/barium hexaferrite nanocomposites were synthesized via a citrate solegel combustion method to prepare
* Corresponding author. E-mail address: [email protected] (J.-Y. Lee). https://doi.org/10.1016/j.matchemphys.2017.10.048 0254-0584/© 2017 Elsevier B.V. All rights reserved.
microwave-absorbing material. P. Latko et al. [2] showed that polyamide 11/multi-walled carbon nanotube nanocomposite fibers were prepared and they found that the nanocomposite fiber could be used as a precursor for non-woven or woven fabrics to manufacture EMI shielding material. X. Shui and D. D. L. Chung [3] reported that polyethersulfone/nickel filament (0.4 mm diameter) composite had high electromagnetic interference shielding effectiveness, high reflection coefficient and low surface impedance at 1e2 GHz. Z. Zhang et al. [4] showed that electrical conductive nanocomposite fibers were prepared with polyaniline (PANI), polyacrylonitrile (PAN) and multi-walled carbon nanotubes (MWCNTs) via electrospinning and the electrical conductivity of nanocomposite fibers increased from 1.79 S m1 to 7.97 S m1 with increasing the MWCNT content from 3.0 wt% to 7.0 wt%. To deposit metallic particles on the plastic surface, metallization processes were carried out through physical deposition method or wet chemical deposition method [8e12]. A. M. Abdel Reheem et al.
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[8] reported on the copper films deposited onto polycarbonate (PC) substrate after surface modification using low argon ion beam and Z. Sun et al. [9] showed an electroless nickel plating method deposited on a primer-printed polyethylene terephthalate (PET) surface by immersing in 3-aminopropyltriethoxysilane solution. S. Olivera et al. [10] showed that a plating technique on acrylonitrilebutadiene-styrene (ABS) surface can serve to enhance the strength and structural integrity as well as to improve durability and thermal resistance, and H. N. Zhang et al. [11] reported that copper was plated on the ABS surface in an electroless copper solution, in which the ABS surface was previously modified by heterocyclic organosilane self-assembled film. Wet chemical deposition methods are usually classified into two categories: electroplating process and electroless plating process. However the former can't be applied to polymer substrates because polymers can't pass electric current. Therefore polymer materials are deposited by using an electroless plating process in general. Prior to electroless deposition, the polymer surface is pre-treated by physical and chemical modification methods and there are many pre-treatment methods: chemical, plasma, laser and catalytic activation methods [12], and in this study catalytic activation method was used. The most commonly used catalyst is palladium (Pd) nanocatalyst because it has good activation property compared to other catalysts such as copper, gold, silver or aluminum nanocatalysts [13]. To activate the polymer surface with Pd nanocatalyst, a combination of Sn and Pd chemistry process is widely used and there are two main processes: one is a two-step process which uses successive dilute solutions of SnCl2 and then PdCl2 [14,15], and the other is a one-step process which uses a SnCl2ePdCl2 mixed solution [14,16]. In recent the latter one, so-called Pd/Sn mixed catalyst is most widely used process in which the mixed catalyst is prepared by mixing PdCl2 with large excess of SnCl2 in HCl solution. Another method to activate the polymer surface with Pd nanocatalyst is to use organometallic compounds such as palladium(II) acetylacetonate (Pd(acac)2), palladium(II) dimethyl sulfoxide (Pd-DMSO), palladium(II) hexafluoroacetylacetonate (Pd(hfa)2), which are dissolved in supercritical CO2 solution [17,18]. We also reported a Pd activation method from Pd(acac)2 on the surface of polymers by using a drying process [19e21]: Firstly, Pd(acac)2 was sublimed and penetrated into polymer surface at 180 C in vacuum condition, and spontaneously reduced forming Pd nanocatalysts. And then Ni-P or ZnO plating was site-selectively deposited on Pd activated surface. In this study PAN fiber surface was activated by Pd nanocatalysts via drying process and Ni-P plated PAN fiber with long aspect ratio was prepared in electroless Ni-P solution. 2. Experiment Pd(acac)2 was purchased from Johnson Matthey Materials Technology, and it was recrystallized in boiled acetone. PAN fiber was obtained from Tai'an Pole Geosynthetics Co., Ltd. (China), whose diameter was 13 ± 1 mm and the length was 15 mm. Its tensile strength was 910 MPa, initial modulus was 100 GPa and density was 1.18 g/cm3. The purchased PAN fiber was cleaned with acetone in order to remove contaminated oil spots during handling and was dried in a vacuum oven at 40 C for 48 h before use. Nickel(II) sulfate hexahydrate (NiSO4$6H2O), sodium hypophosphite monohydrate (NaPH2O2$H2O), propionic acid (CH3CH2COOH), lactic acid (CH3CH(OH)COOH), and ethanol were used as received from Wako Pure Chemical Industries, Ltd. In order to activate the surface of PAN fiber with Pd nanocatalyst, one-step drying process was used [19e21]. As shown in Fig. 1, firstly 0.02 mg of Pd(acac)2 was poured into a 50 mL quartz reactor and it sublimated at 180 C oil bath. The sublimated
Pd(acac)2 recondensed on the upper side of the reactor wall, PAN fiber with ca. 0.5 cm length was put into the reactor, and the atmosphere of the reactor was exchanged with N2. If the reactor containing Pd(acac)2 and PAN fiber was kept at 180 C oil bath, the sublimated Pd(acac)2 precursors penetrated into PAN fiber surface and the Pd(acac)2 compounds were spontaneously reduced to Pd nanoparticles in the PAN fiber surface without any reducing agent at 180 C N2 atmosphere. The exposure time of PAN fiber to Pd(acac)2 was 5 min. To remove the unreduced Pd(acac)2 on the PAN surface, the Pd activated PAN fiber was cleaned in boiled acetone and dried in a vacuum oven at 40 C for 24 h. Electroless Ni-P plating solution was prepared by dissolving 6.3 g of nickel(II) sulfate hexahydrate (Ni source), 7.5 g of NaPH2O2$H2O, 7.5 g of lactic acid (buffer and complexant), and 8.1 g of propionic acid (buffer) in 60 mL deionizer water and pH was adjusted to 4.5 ± 0.1 by NaOH aqueous solution. Then, 30 mL ethanol was mixed with the electroless Ni-P plating solution in order to modify wettability of PAN fiber to the electroless solution. 0.5 g of chopped PAN fiber was put into the electroless solution and Ni-P plating was carried out in the plating solution at 80 C for 10, 30 and 60 min. Ni-P plated fiber was cleaned with distilled water and was dried in a vacuum oven at 40 C for 24 h. The dispersion of Pd nanocatalysts on the surface of PAN fiber was observed by a transmission electron microscopy (TEM, Carl Zeiss Co. Ltd., Germany) at an accelerating voltage of 200 kV. Thin sections for Pd/PAN fibers were prepared by cryo-ultramicrotomy at 60 C after embedding in a light curable resin system (D-800, JEOL DATUM, Japan). The morphology of Ni-P plated fiber was observed by a field emission scanning electron microscopy (FESEM, JMS-6701F, JEOL) which was operated at an acceleration voltage of 10 Kv. The elemental analysis was carried out by energydispersive X-ray spectroscopy (EDS, JED-2300 Energy Dispersive Xray Analyzer, JEOL) which was connected to the FE-SEM. X-ray photoelectron spectroscopy (XPS) spectrum was acquired by Thermo Scientific K-Alpha model equipped with 180 double focussing hemispherical analyzer using monochromated Al KAlpha radiation. To measure the tensile strength of a single PAN fiber or a single Ni-P plated fiber, universal testing machine (Shimadzu, Table top type tester EZ-Test) was used with a load cell of 10 N and the crosshead speed was set to 0.2 mm/min. The specimen was prepared by fixing a filament on a paper holder with an instant adhesive, as reported on the other paper [22], and its diameter was measured by using an optical microscope. EMI shielding effectiveness (EMI-SE) of Ni-P plated PAN fiber/ epoxy composite was assessed using coaxial transmission line method according to ASTM D4935 [23]. The composite specimen was prepared by curing the mixture of Ni-P plated PAN fiber (50 wt %) and epoxy matrix at 150 C for 2 h. The Ni-P plated PAN fiber used here was plated at 80 C for 60 min. 3. Results and discussion Fig. 1 shows a TEM image for Pd nanocatalysts activating the PAN fiber surface prepared in 180 C N2 atmosphere for 5 min. Almost all Pd nanocatalysts were located on the surface area of the PAN fiber so that the Pd nanocatalysts could act as nucleus for Ni-P growth. To confirm the formation of Pd nanocatalysts through the reduction of Pd(acac)2, Pd activated PAN fiber was analyzed by EDS and the spectrum was shown in Fig. 2. The peaks for carbon (C) and nitrogen (N) were due to PAN fiber and Au peak was owing to gold coating for SEM observation. There were only Pd peaks without oxygen (O) peak which meant all Pd(acac)2 compounds spontaneously reduced to metallic Pd nanocatalysts. The formation mechanism of metallic Pd nanocatalysts in PAN fiber was divided
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Fig. 1. One-step drying process for the preparation of Pd activated PAN fiber.
Fig. 3. (a) SEM image of Pd activated PAN fiber prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min and (b) EDS data.
Fig. 2. Pd nanocatalysts on the surface of PAN fiber prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min.
into two steps: the first step was nucleation step and the second one was growth step in the PAN fiber surface area. In the nucleation step, Pd(acac)2 as metallic precursors sublimed in 180 C N2 atmosphere, penetrated into the surface of PAN fiber, and the precursors adsorbed on nucleophilic C≡N groups of PAN fiber. And then the precursors spontaneously reduced to Pd elements forming nanocatalysts as nucleation sites. And in the Pd growth step, Pd precursors continuously adsorbed on the nucleation sites and reduced to Pd element resulting fast increasing Pd nanoparticles. In this work, however, Pd nanocatalysts were positioned at the surface area of PAN fiber because PAN fiber was too dense for vaporized Pd(acac)2 to penetrate deeply into the PAN fiber, and it was advantageous to electroless plating. Fig. 3 shows SEM images in order to observe Ni-P morphology plated on PAN fiber surface which was prepared in electroless plating solution at 80 C for 30 min: (a) 350x, (b) 1,600x and (c)
6,500 magnification. As was expected, 0.5 cm PAN fiber was evenly plated with Ni-P alloy as shown in Fig. 3(a), and when the surface was zoomed in nickel clusters grew along the furrows of PAN fiber as shown in Fig. 3(b) and (c). Because surface roughness of polymer affect the adhesion strength between plated layer and the polymer surface, the furrows on the PAN fiber, as shown in Fig. 2(a) would have good effect on the adhesion strength. The electroless mechanism of Ni-P plating could be divided into four elementary steps [24]: (1) reactants (Ni2þ, H2PO 2 etc.) diffused to the surface of PAN fiber, (2) adsorpted on Pd nanocatalysts on the PAN surface, and (3) Ni-P plating reaction took place on the PAN þ surface, and then (4) the by-products (HPO 3 , HI, H etc.) desorpted and diffused from the PAN surface. In these steps, the first step was rate determining step because the surface of PAN fiber was
Fig. 4. SEM images of Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 30 min: (a) 350x, (b) 1,600x and (c) 6,500 magnification.
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hydrophobic, therefore ethanol was mixed into the conventional Ni-P plating solution in order to improve the wettability of PAN fiber to the plating solution in order to increase the reaction rate of the first step. Fig. 4 shows EDS spectrum for Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 30 min. The morphology of
Fig. 5. (a) SEM image of Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 30 min and (b) EDS data.
Fig. 6. XPS survey spectra for Pd activated PAN fiber prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min and Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for 10 min.
Ni-P plated PAN fiber was observed by SEM as shown in Fig. 4(a) and the elemental composition was analyzed by EDS as shown in Fig. 4(b) resulting the elemental composition of Ni: P was 94.3: 5.7 atomic %. There was no atomic peak for nitrogen or carbon atom owing to PAN fiber and this meant the surface of PAN fiber was wholly covered by Ni-P alloy. Fig. 5 shows XPS survey spectra for Pd activated PAN fiber and for Ni-P plated PAN fiber. Pd activation of PAN fiber was prepared by spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min and Ni-P plated PAN fiber was prepared at 80 C for 10 min in electroless plating solution. In the spectrum of Pd activated PAN fiber, if displayed in detail, the binding energy (BE) values of Pd3d peaks would be shown at 341.1 eV and 335.5 eV [20], and the BE values of C1s and N1s were 286.2 eV and 398.0 eV, respectively for the nitrile group of PAN fiber [25]. This XPS spectrum confirmed that all the penetrated Pd(acac)2 compounds were reduced to Pd metallic nanoparticles because there was no O1s peak owing to Pd(acac)2 compound. However, in the spectrum of Ni-P plated PAN fiber, the BE values of C1s and N1s were reduced very significantly and new peaks for nickel (855.7 and 870.7 eV) and phosphorus (135.2 eV) elements. These meant that almost all the surface of PAN fiber was covered by Ni-P alloy, and a weak peak of C1s was due to the carbon atoms at the open hole on the PAN surface as shown in Fig. 6(a). As Ni-P plating time was 30 min, the peak of C1s was completely disappeared, although its XPS spectrum was not displayed in Fig. 5, and this meant that all the surface of PAN fiber was completely covered by Ni-P deposition as shown in Fig. 6(b). The effect of electroless plating time on Ni-P thickness was studied by using SEM observation shown in SEM images in Fig. 6, in which each Ni-P was plated on PAN fiber in electroless plating solution at 80 C for (a) 10, (b) 30 and (c) 60 min, and each prime (’) image was prepared by thermal decomposition of each Ni-P plated fiber at 600 C for 30 min in N2 atmosphere condition in order to measure Ni-P thickness. As was reported by others [26,27], Ni-P particles generated irregularly on the PAN surface and grew gradually. Almost all the PAN surface was covered in 10 min reaction as shown in Fig. 6(a), and as electroless plating time increased, the thickness of Ni-P increased and the morphology of Ni-P surface became rough. The Ni-P thickness in 10 min was 0.64 mm and those in 30 and 60 min were 1.03 and 1.22 mm, respectively. The growth rate of Ni-P layer decreased with increasing plating time. Tensile strength of a single PAN fiber was 918.6 MPa and its initial modulus was 125.7 GPa, which were was in good agreement
Fig. 7. SEM images of Ni-P plated PAN fiber prepared in electroless plating solution at 80 C for (a) 10, (b) 30 and (c) 60 min. Each prime (’) image was prepared by thermal decomposition of Ni-P plated PAN fiber at 600 C for 30 min in N2 atmosphere condition in order to measure Ni-P thickness.
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time was 10 min, almost all the PAN surface was evenly covered by Ni-P alloy with some open holes and the holes were completely filled as the plating time became 30 min. Ni-P alloy thickness was 0.64, 1.03 and 1.22 mm in the reaction time of 10, 30 and 60 min, respectively. EDS analysis showed that elemental composition ratio of Ni: P was 85.3: 14.7 atomic %. EMI-SE of Ni-P plated PAN fiber (50 wt%)/epoxy composite was more than 30 dB in the frequency range from 30 MHz to 1500 MHz, therefore the composite could be used for EMI shielding materials.
Acknowledgement This work was financially supported by the Regional Innovation Center (RIC) program of the Ministry of Trade, Industry and Energy (MOTIE No. R0010583), Korea in 2016 and by the Ministry of Education (MOE No. 2017CG0260101) through the Leades INdustryuniversity Cooperation Project.
References
Fig. 8. EMI shielding effectiveness of Ni-P plated PAN fiber (50 wt%)/epoxy composite where the Ni-P plated PAN fiber used here was plated at 80 C for 60 min.
with the specifications provided by the manufacture. Those values for Ni-P plated PAN fiber, which was prepared at 80 C for 60 min were 895.2 MPa and 138.4 GPa, respectively. Generally, when the metal was coated, the tensile strength increased, however the value slightly decreased in here. This was probably because the structure of the electrolessly plated Ni-P was not dense, as shown in the SEM image of Fig. 7. Fig. 8 showed the EMI-SE of Ni-P plated PAN fiber (50 wt %)/epoxy composite where the Ni-P plated PAN fiber used here was plated at 80 C for 60 min. It showed that the EMI-SE was more than 30 dB in the frequency range from 30 MHz to 1500 MHz. Therefore, the composite could be practically useful for many applications requiring EMI shielding [23]. 4. Conclusions Ni-P metallized PAN fiber was prepared by using electroless plating process, in which the surface of PAN fiber was previously activated by Pd nanocatalyst via the spontaneous reduction of Pd(acac)2 in 180 C N2 atmosphere for 5 min. TEM observation showed that almost all Pd nanocatalysts were located on the surface area of the PAN fiber and SEM observation showed that Ni-P alloy clusters grew along the furrows of the Pd activated PAN fiber surface. As plating time increased, the Ni-P layer thickness became thick while the growth rate decreased. When Ni-P plating
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